1. Field of the Invention
The present invention relates to integrated magnetic transducers and it is particularly applicable to the reading of information contained on magnetic data supports such as magnetic discs, rigid or flexible tapes and the like.
2. Description of the Prior Art
It is known that magnetic discs may have information recorded thereon on registering paths which are concentric and circular and which have a radial length not exceeding several hundredths of millimeters and generally covering the major partof their two faces. Magnetic tapes on the other hand carry information on longitudinal paths parallel to the length of the tape.
Integrated magnetic transducers are being utilized more and more frequently either for registering information on registry supports such as discs or magnetic tapes or to read information previously recorded thereon. Generally, one or several transducers are associated with the registry support which is transported before the one or the several transducers. For simplification, it will be supposed in the following description that a single transducer is associated with a single registry support.
For a better understanding of the objects and advantages of the present invention, a brief mention will be made of the several theories of magnetism.
To magnetize a magnetic material within its interior when the magnetic induction is weak, the material should first be subjected to a magnetic field having a sufficient intensity so that the material is saturated, that is to say, that the magnetic induction in the material reaches a limit value B.sub.s. The exterior magnetic field is then removed. There then remains in the material a magnetic induction of some value called remanant induction, B.sub.r, characteristic of the material. The initial magnetic permeability of the material is defined by the ratio (B/H) between the induction and the magnetic field when B and H are close to zero and this at the starting point of the hysteris curve of magnetization. In other words, the initial magnetic permeability of the material is equal to the slope of the magnetic curve in the neighborhood of point B=0 and H=0.
It will be recalled that an anisotropic magnetic material has in general two privileged directions of magnetization which are most often perpendicular one to the other. One of these is called direction of easy magnetization and the other is called direction of difficult magnetization. The initial permeability of the material in the direction of difficult magnetization is much greater than the initial permeability of the material in the direction of easy magnetization. Integrated magnetic transducers of the type to which the present invention relates are described, for example, in U.S. Pat. No. 3,723,665 and U.S. Pat. No. 3,846,842.
Such a transducer comprises a magnetic circuit formed of two polar pieces in thin layers, connected at one end in such a way that they are magnetically coupled and disposed with its other end adjacent the registry support associated with the transducer in such a way as to form an air gap. One of the two polar pieces is disposed on a substrate. Each polar piece has a substantially rectangular form with its largest, or longitudinal dimension being perpendicular to the registry support. The air gap has a substantially rectangular shape and its large dimension Lpm is on the order of the radial size of the paths of the support, and its plane is substantially perpendicular to the polar pieces and disposed at a very small distance from the surface of the registry support, a distance being between zero and several tenths of microns. An electric coil formed of thin conductive superimposed layers. These layers are separated from each other by thin electrically insulating layers. Reference to thin layers means layers having a thickness on the order of several Angstroms to several microns. A part of the coil passes between the two polar pieces in such a way that these polar pieces form an envelope for this part of the coil.
The magnetic registry support associated with the transducer is transported in front of the transducer and perpendicular to the plane of the polar pieces and perpendicular to the greatest dimension of the gap. It is known that the magnetic information on a path of a registry support produces a magnetic flux loss at the immediate neighborhood of the surface of the support. This flux which passes through the air gap of the integrated magnetic transducer with which it is associated traverses the coil of the transducer producing a useful read signal S that is received at the poles of the coil. This signal is sent to electronic circuits for reading associated with the transducer.
In current practice, the magnetic material making up the polar pieces is preferably anisotropic and its direction of easy magnetization is parallel to the registry support and equally parallel to the large dimension of the air gap. The axis of difficult magnetization has the same direction as the large dimension of the polar pieces, i.e., perpendicular to the registry support.
Preferably, the integrated magnetic transducers of which the polar pieces are constituted of an anisotropic magnetic material are made in the manner described in U.S. Pat. No. 4,016,601. An integrated magnetic transducer such as that described in U.S. Pat. No. 4,016,601 comprises polar pieces which have a narrowing at the level of the air gap. The length of the narrowing measured in the plane of the gap is equal to the large dimension of this last or Lpm, and is also designated under the name of "width of the geometric path". To obtain this narrowing, the polar pieces are machined by ionic attack on a depth of attack normal to the plane of the gap and equal to the length Prof (Prof, which is on the order of several microns, is measured in the direction of the large dimension of the polar pieces). Such integrated magnetic transducers have a relatively high reading efficiency which is a measure of the ratio between the voltages available at the poles of the winding and the magnetic flux which enters in the transducer at the level of the air gap.
Finally, to better understand the objects and advantages of the present invention, it is useful to consider the following: A polar piece is constituted by a single or by several layers of magnetic material. In one, as in the other of the two cases, the polar piece is defined as "single layer polar piece" if the direction of magnetization can change, in discontinuous manner, only through a magnetic wall and not through a layer deposited or formed by oxidation or other processes. The magnetic wall is called the geometric location of the points of the single layer polar piece where the change of direction occurs, the phenomena being considered on the macroscopic scale. It is thus seen that one considers the definition of the "single layer polar piece" under its magnetic aspect and not its geometric and/or crystalline aspect.
In view of the dimensions of each polar piece, it may be observed that there is created by reason of all of the responsible factors of its anisotropie, a plurality of small magnetic areas among which some exist where the magnetization is anti-parallel which means that for two adjacent magnetic areas the magnetization is in opposite sense. A direction of magnetization in each of the magnetic areas is parallel to the direction of easy magnetization, that is to say parallel to the registry support and perpendicular to the direction of movement of it. This phenomena of appearance of magnetic areas in the anisotropic single layer polar pieces of small dimension is established by M. Jean-Pierre LAZZARI in his doctorate thesis at Grenoble on Dec. 18, 1970 having the title: "Studies and Structures of a New Registry Head: Integrated Magnetic Head in Thin Layers" and also described in the following publication: Hempstead R. D., Thompson D. A., Transaction on Magnetics, Vol. MAG 14, No. 5 Sept. 78, of which the title is: Unidirectional Anisotropy In Nickel-Iron Films by Exchange Coupling With Antiferromagnetic Films". The geometric location of the points separating two magnetic areas which are adjacent or where the magnetization changes in sense and direction is called by analogy the magnetic wall with the definition given above of the monolayer polar piece. It is also shown that outside of the adjacent magnetic areas with anti-parallel magnetization, there also exist so called closed areas situated on the edges of each polar piece. There occurs in effect between two adjacent magnetic areas with anti-parallel magnetization on the edges of the polar pieces small closed magnetic areas where the magnetization has a different orientation from that of the magnetization in these two magnetic areas with anti-parallel magnetization.
When the integrated magnetic transducer reads this informations, the existence of the magnetic areas with anti-parallel magnetization and of the closed magnetic areas causes the existence of a Barkhausen noise signal which is superposed on the useful output signal S of the transducer. This is due to the following phenomenon: Under the effect of the magnetic field of loss of information of the support, the magnetic walls separating two areas displace. There occurs the creation of an electro-motive induction force of noise, called "parasitic induction force" due to the modification of the distribution of the magnetization in the pole pieces caused by the displacement of the magnetic walls. It is shown that this Barkhausen noise signal takes on greater importance as the ratio Prof/Lpm becomes greater and that is to say when Lpm is small or small with respect to Prof. It can be shown that in this case, the number of magnetic areas is large. The Barkhausen noise signal is a function of the number of areas.
Thus, the Barkhausen noise signal is large when, for example, the information is to be read from a magnetic disc of which the radial density of registration is high. Radial density is defined as the number of registry paths per millimeter measured along a diameter of the disc. Because the radial width has a small value, the value Lpm is equally very small consequently the ratio Prof/Lpm is large.
It should thus be apparent that for an integrated magnetic transducer having the value of Lpm small with respect to Prof (and in front of the large dimension of the polar pieces), there is: On the one hand an increase in the noise signal due primarily to the increase of the Barkhausen noise signal, which signal is called B; and on the other hand a decrease of the useful signal S since in the first approximation this is proportional to Lpm. Consequently the ratio signal/noise S/B decreases.